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CCM published two review articles in “Reports on Progress in Physics”

Recently, Prof. Huiqiu Yuan’s team from the Center for Correlated Matter and Department of Physics at Zhejiang University published two review articles in the prestigious journal Reports on Progress in Physics (RoPP). Published by the UK based Institute of Physics, RoPP “publishes review articles covering all branches of physics, written by invited authors who are worldwide experts in their field”. The first review article is part of a special issue on strongly correlated electron systems and is about multiple quantum phase transitions and superconductivity in Ce-based heavy fermions [Z. F. Weng et al., Rep. Prog. Phys. 79, 094503 (2016)], while the second one is about superconductivity and spin-orbit coupling in noncentrosymmetric materials [M. Smidman et al., Rep. Prog. Phys. 80, 036501 (2017)]. The first article is written by scientists from the Center for Correlated Matter and Department of Physics, with Xin Lu and Huiqiu Yuan as corresponding authors. In the second review article, Michael Smidman and Huiqiu Yuan (co-corresponding author) from Zhejiang University and Myron Salamon from the University of Texas at Dallas were responsible for the experimental part, while Daniel Agterberg (co-corresponding author) from the University of Wisconsin was responsible for the theoretical part. Publication of these two invited review articles in RoPP highlights the significant contributions of scientists from the Center for Correlated Matter to the study of heavy fermions and noncentrosymmetric superconductors.

Heavy fermion compounds generally contain rare earth or actinides elements which have partially filled f-electron shells. These f-electrons can hybridize with conduction electrons, leading to the quenching of the f–electron magnetic moments. In heavy fermion materials there is competition between this quenching and the tendency for spins to align or anti-align in an ordered pattern, which leads to a wide range of unusual behaviors, including unconventional superconductivity and quantum phase transitions. As well as giving an overview of the unconventional superconductivity found in several cerium based heavy fermion systems, the different methods for tuning heavy fermion states are also discussed in the first review article. Due to the relatively low energy scales, the ground states of heavy fermion systems are often readily tuned by non-thermal parameters. Scientists at the Center for Correlated Matter have made significant contributions to this field. For instance, Huiqiu Yuan et al. observed two distinct superconducting states in the heavy fermion superconductor CeCu2Si2 upon applying pressure, where superconductivity in the low-pressure region takes place on the threshold of magnetism and the high-pressure superconducting state appears near a possible instability of the cerium valence [Science 302, 2104 (2003), Phys. Rev. Lett. 96, 047008 (2006) and New J. Phys. 6, 132 (2004)].  These findings solved the long-standing puzzle of the unusual pressure-temperature phase diagram of CeCu2Si2. Recently, evidence for a crossover from mixed valence to heavy fermion behavior was also observed in Yb2Ni12(P/As)7 under pressure [Sci. Rep. 5, 17608 (2015)]. By combining the effects of pressure, magnetic field and doping, Xin Lu and Huiqiu Yuan’s group (together with collaborators at Los Alamos) have shed new light on the interplay of superconductivity and quantum criticality in the CeTIn5 compounds [Phys. Rev. Lett. 114, 146403 (2015), Phys. Rev. B 89, 041101(R) (2014), Nat. Phys. 10, 120 (2014), Nature 440, 65-68 (2006)]. In some cases, the magnetic transition temperature can be continuously suppressed to zero temperature at a quantum critical point (QCP) by applying various tuning parameters, where it is quantum rather than thermal fluctuations which dominate the physical properties. It was recently demonstrated that different types of QCP may be realized in the heavy fermion compound CeRhIn5 upon either applying pressure or magnetic fields, which may be universally classified by how the Fermi surface changes at the QCP [PNAS 112 ,673 (2015)].

Most superconductors have a center of inversion and as a result the Cooper pairs can be classified as either spin singlet or spin triplet. However, when spatial inversion symmetry is broken in a superconductor, Cooper pairs are no longer entirely singlet or triplet, but are a mixture of the two which allows for the study of novel and unusual superconducting behavior. The second review article gives an extensive and comprehensive review of both the theoretical and experimental aspects of noncentrosymmetric superconductors. Huiqiu Yuan’s group is one of the leading research groups in the world on the study of noncentrosymmetric superconductors. They have examined the pairing state of numerous noncentrosymmetric superconductors with various spin-orbit coupling strengths and published a series of articles on this subject. For example, they provided the first evidence for an s-wave spin-triplet state in Li2Pt3B due to enhanced spin-orbit coupling [Phys. Rev. Lett. 97, 017006 (2006)]; reported the first experimental evidence for nodal superconductivity in the quasi-one-dimensional compound K2Cr3As3 [Phys. Rev. B 91, 220502(R) (2015)] and proposed a new even-parity spin-triplet pairing state for LaNiC2 and LaNiGa2, where time reversal symmetry is broken below the superconducting transition  [Phys. Rev. Lett. 117, 027001 (2016)].

These works were supported by National Natural Science Foundation of China, National Key Research and Development Program of China  and the Science Challenge Project of China.